In VDSL systems the transmitted signal passes through a selective channel that severely affects the communication quality.
One of the modulation proposed for VDSL is the well-known multi-carrier approach of DMT (Discrete Multi Tone) modulation. In DMT where the available bandwidth is split into many sub-bands (normally 2048 or 4096) identified with a respective typical subcarrier frequency.
An aspect of a multi-carrier approach like DMT is that a selective channel is converted to a flat one, with obvious advantages for the communication. DMT system derives from OFDM modulation (Orthogonal Frequency Division Multiplexing) and is improved with respect to the latter owing to a proper bit-loading algorithm. This algorithm provides that starting from the measurement of the SNR (Signal to Noise Ratio) on each subcarrier, the right number of bits to be sent is derived for each of them. The general assumption of the loading operation is that the higher the SNR, the higher the QAM (Quadrature Amplitude Modulation) order, and as a consequence the higher the number of bit carried, keeping the SER (Symbol Error Rate) level under a certain reference value, typically 10−7 for xDSL application.
Another modulation approach is CDMA (Code Division Multiple Access), which has a better spreading gain against interference and against “noise”, and furthermore eases the system synchronization and robustness against frequency and phase offset or jitter.
A natural join between DMT (OFDM) and CDMA is Multi-Carrier CDMA. There are three possible ways to combine CDMA and multi carrier transmissions schemes:                Multi Carrier (MC) CDMA [4], which spreads the original data stream over different subcarriers using a given spreading code in the so-called frequency domain;        Multi Carrier DS-CDMA [5], which spreads the S/P converted data stream, using a given spreading code in the time domain, satisfying the orthogonality condition;        Multi Tone (MT) CDMA [6], which spreads the data stream in the time domain, but there is partial overlapping between the frequency bands reserved to each branch out of S/P converter after spreading, causing the resulting spectrum not to satisfy the orthogonality condition anymore.        
Moreover, a MC CDMA scheme with MMSE (Minimum Mean Square Error) is known [6] providing a 2 path frequency selective slow Rayleigh fading channel.
It must be noted that the above multi-carrier CDMA modulation schemes have not been used for a VDSL application, but only for wireless application, nor their suitability to a wired system has been proposed yet.
The way to join Multi-Carrier transmission like OFDM and DMT and CDMA modulation raises the following general considerations:                on one hand OFDM modulation is robust to frequency-selective fading, but suffers in subcarrier synchronization and is sensitive to frequency offset and nonlinear amplification, which results from the fact that it is composed of many subcarriers with overlapping power spectras and exhibits a non-linear nature in its envelope [4].        On the other hand, CDMA systems are quite robust to frequency offsets and nonlinear distortion.        The combination of OFDM signaling and CDMA scheme lowers the symbol rate on each subcarrier so that a longer symbol duration, in the time or frequency domain, eases the system synchronization [4]        performing timing spreading (MT-CDMA and MC-DS CDMA) calls for extra hardware blocks in order to control and synchronize the despreading operation on the receiver front-end (for instance a code acquisition and tracking loop is necessary).        the use of MC-CDMA avoids the need of extra hardware, except from the de-spreading blocks.        in MC-CDMA the spreading/despreading operation can be easily accomplished in the frequency domain before the IFFT processor on the transmitter side or after the FFT processor on the receiver side.        
However, some drawbacks affect the use of MC-CDMA as such in a VDSL modem:                using a code division modulation results in a decreasing of the achievable channel rate, since the number of subcarriers available for transmission is reduced by a factor equal to the spreading gain;        usually a CDMA scheme offers a code division multiple access to a shared resource, and it can also manage heavy Multiple Access Interference (MAI) interference with precision. However, in a VDSL environment the MAI interference is arising from an electromagnetic coupling mechanism among twisted pairs, thus the resulting interference to useful signal (I/S) power ratio is so low before despreading that no remarkable results can be achieved after despreading. In this way, the use of a CDMA to remove the system crosstalk interference (NEXT, FEXT) is inappropriate.        